Labeling products by means of freely programmable labeling systems has been known for a long time, and is used industrially in many areas. For example, ink-jet printers, among other things, are used for a plurality of applications. They allow labeling of products without contact, in that ink droplets are ejected from one or a plurality of nozzles of a print head, in such a manner that a print image in the form of a dot matrix can be built up on the surface to be imprinted.
In this connection, essentially two technologies are known: continuous ink jet and drop-on-demand technology.
In the continuous ink jet technology, an ink jet that exits continuously, in most cases from a single nozzle, is modulated by means of a modulation element for producing print variations, for example by means of a piezo oscillator, in such a manner that the jet breaks up into individual droplets having essentially the same size after it exits from the nozzle at a certain spacing from the nozzle. The ink droplets formed in this way can be charged electrostatically with different charges immediately before they separate from the ink jet, and can then be deflected by an electrical field of a downstream plate capacitor to an extent related to their respective electrostatic charges, thereby resulting in different flight paths for the ink droplets at least in terms of one deflection direction.
In this way, it is possible to select different positions of the ink droplets that have different charges at least in a direction crosswise to the direction of spread of the ink droplets, thereby making it possible to produce a print image composed of dots if a substrate to be labeled is moved simultaneously and synchronously relative to the print head.
Since this working principle also functions with a significant spacing between the print head and the surface to be imprinted and with good image good quality, it is also possible to label moderately textured surfaces, for example. However, it is a disadvantage that the maximal height of a print line produced in this way can amount to only approximately 10 mm to 20 mm, due to system conditions, and therefore it is not possible, for example, to produce a larger imprint with only a single print head, or to label of a pipe or cable, for example, in such a manner that the outer surface of the pipe or cable can be imprinted in an angle range of more than 120°, or actually in an angle range of 360°.
In these cases, it is necessary to use multiple print heads and/or printing systems, and to mount them, for example in a circle around the object to be imprinted for 360° imprinting, the print image being divided up among the number of printing systems used that must be operated synchronously. Of course, such a method is complicated, error-prone, and expensive.
In contrast to the continuous ink-jet method, a print head of a drop-on-demand printing system has a plurality of individual nozzles that can be controlled independently of one another. The nozzles are generally oriented in a line in a nozzle plate, and have the same spacing from one another, in each instance.
The production of individual ink droplets takes place, according to this principle, in that each nozzle, together with a respective nozzle chamber and a respective modulation element forms a separate print module that can be controlled independently. For this purpose, all the nozzle chambers of a print head are connected with a common ink reservoir by way of common ink feed lines, for example, so that in operation the nozzle chambers are all filled with ink.
In this connection, the modulation elements are mounted on the respective nozzle chambers such that they can build up an excess pressure in the respective nozzle chambers in pulse-like manner, where corresponding electrical control occurs, thereby causing an ink droplet to be expelled to the outside from the respective nozzle. Because the print modules can essentially be controlled independently of one another, it is possible to produce a complete print line with a single common control pulse, where the control pulse triggers all the required nozzles at the same time, by way of corresponding control circuits.
Depending on the type and embodiment of the print heads, the print chambers are connected with one another by way of the above-described ink feed line, and furthermore the nozzles can be controlled to produce droplets only in specific groupings, depending on the method of operation of the modulation elements, in that the nozzles 1, 4, 7, . . . are turned on with a first print pulse, for example, the nozzles 2, 5, 8, . . . are turned on in a second print pulse, and the nozzles 3, 6, 9, . . . are turned on in a third print pulse, thereby resulting in a saw-tooth-like print line when an object to be imprinted moves continuously past the print head.
As a condition of the system, it is not possible to produce high-quality imprinting over a greater distance, for example on a curved or textured surface of a product, using drop-on-demand print heads, since the ink droplets produced using this method have a relatively low ejection speed and a small diameter, and therefore a small mass, and thus can be deflected from their flight path in uncontrolled manner, after only a short flight distance, due to air resistance and external air movements such as those that result from movement of the object to be imprinted, and therefore a deterioration of the print image occurs that increases with spacing.
However, since it is generally desired to achieve high print quality, it is furthermore necessary to make available a large number of print channels, and, at the same time, the spacing between adjacent nozzles must be small, and the ink droplets that are ejected must be small. Technically, this is accomplished in that a complete print head is built up more or less monolithically, in that all the nozzle chambers, their corresponding ink feed lines, and the modulation elements are combined in a common base body, for example, and form a rigid block. Such print heads that work according to the drop-on-demand method are manufactured and sold by the companies Spectra, Xaar, Konika Minolta, Hewlett Packard, or Epson Industrial, for example.
However, it is a disadvantage of the above-described print heads that surfaces having a significant curvature can only be imprinted unsatisfactorily, or not at all, in a single work step, or only using a plurality of print heads, and this means a high investment.
In German patent document 40 33 816, in contrast, a flexible print head is described with which it is possible to imprint a curved surface within over part of its curvature, but the shape of the print head is adjustable only as an arc over a fixed center point, and thus only with a curvature in one plane, and therefore it is not suitable for imprinting surfaces having a different shape. Furthermore, 360° imprinting is not possible with such a print head.